Stents are tubular medical devices which are indwelled in a lumen in order to expand a constricted part or an obstructed portion and secure the lumen in order to treat various diseases caused when blood vessels or other lumens in a living body are constricted or obstructed. Since the stent is inserted into the body from the outside of the body, the stent has a small diameter at the time of insertion, the stent is expanded or restored at a targeted constricted part or obstructed part to make its diameter larger, and a lumen is held in an enlarged state. Stents are sometimes classified as self-expandable stents and balloon expandable stents according to the function and expansion mode of the stent. The balloon expandable stent has no expansion function in the stent itself (i.e., no self-expanding function). After the stent is inserted into the targeted part, a balloon is located in the stent, the balloon is expanded, and the stent is expanded (plastically deformed) by the expansive force of the balloon and is brought into close contact with and fixed to the inner surface of the targeted lumen.
In the self-expandable stent, a number of shapes are suggested and are actually used for patients. The initial types of self-expandable stents included one, disclosed in Japanese Patent Publication No. 4-32662 obtained by bending a wire to utilize the repulsive force thereof, and another one described in Japanese Patent No. 2735795) obtained by knitting a wire into the shape of a blade. These stents had difficulties in fine working since the wires are bent, and the blade-shaped stent had a problem in that the length of the stent differs greatly before and after being indwelled in the body.
The present applicant has also suggested a stent as described in Japanese Application Publication No. 8-000738 in which a superelastic pipe is cut out and manufactured by a laser or the like. This technique enhances the working accuracy and enables the stent to form various shapes. However, in the basic structure of the stent, a plurality of wavy-line annular bodies is arranged in the axial direction, and adjacent wavy-line annular bodies are connected together by a connector.
Additionally, the stent disclosed in Japanese Application Publication No. 11-505441 is characterized in that a connector is connected diagonally.
Additionally, as another structural characteristic, the apexes of adjacent wavy-line annular bodies enter the adjacent wavy-line annular bodies. A stent of this type is the arrowhead type stent disclosed in JP-T-2000-506753. In JP-T-2002-518087, the apexes of wavy-line annular bodies are connected by a connector parallel to an axis. In Japanese application Publication No. 2001-137353, the apex with the apex of wavy-line annular bodies and the intermediate portion with the intermediate portion of wavy-line annular bodies are connected by a connector.
Additionally, there is a stent in which a wavy-line annular body which constitutes the stent is not an annular body but a spiral body, and the stent is constituted by one or a plurality of the spiral bodies from a front end of the stent to a rear end of the stent. An example of a stent of this type is one in which zigzag elements are connected together by a connector parallel to an axis in order to maintain the shape of a stent such as disclosed in JP-T-2001-509702. Additionally, as shown in Japanese Application Publication No. 8-196642, the present applicant has suggested a stent constituted by a plurality of spirals.
In many cases, the diameter after expansion of the self-expandable stent is 6 to 12 mm. The stent is housed in a tube called a sheath before expansion. The sheath generally has an external diameter of about 2 mm and an internal diameter of about 1.6 mm. For this reason, the stent is self-expanded from 3.75 times to 7.5 times on an external diameter basis when discharged from the sheath.
Additionally, in a stent for a coronary artery used widely as a balloon-expandable stent, the external diameter of about 1.2 mm is increased from about 2.5 mm to about 4 mm by the balloon. For this reason, this stent is expanded from 2.1 times to 3.3 times on an external diameter basis by the balloon.
As described above, since the self-expandable stent has a higher expansion ratio than the balloon expandable stent, the gap between elements after the expansion tends to increase. Many gaps are a factor which worsens the coverage, and may cause tissue growth between the gaps, which causes restenosis. Additionally, when the line width of each element is made small in order to improve coverage, the expansive force becomes weak, and thus, a certain line width is required.
In all the recent stents described above, a connector connects the respective wavy-line annular bodies together. The connector functions simply to connect the wavy-line annular bodies together.
The stent should also desirably be pliable in the axial direction. If the stent is hard in the axial direction, upon inserting the stent into a lumen, such as a bent blood vessel or biliary duct, there is a possibility that the stent may tend to straighten out the bend, and the tissue may be stimulated at the both ends of the stent and cause restenosis. An example of a stent of a type in which wavy-line annular bodies are independent and are connected together by a connector is the stent in Japanese application publication No. 11-262531. A stent such as this tends to be relatively hard in the axial direction.
Meanwhile, in a stent of a type where the apexes of wavy-line annular bodies are adjacent to each other between wavy-line annular bodies, for example the stent in JP-T-2002-518087, each element is long and apexes of adjacent elements are included.